Br2 and light homolytically cleaves the Br2 bond to give you two bromine radicals. These can react with alkenes and such, typically in the fashion that gives you the bromine at the least substituted position, with the resulting radical at the most substituted position, which is more stable. The radical formed in this step can undergo other propagation steps such as H atom abstraction, or combine with another Br radical to terminate the chain.
NBS should react in much the same fashion, except usually it's used as a Br+ source, and not a Br radical source, although it can do both. However, I would imagine you'd need to heat it to force the NBS to fall apart and give up the Br radical. Either way, it's still a source of Br radical and should exhibit the same reactivity.
HBr can react with alkenes in an ionic mechanism with alkenes, or, the reaction can be initiated with peroxide to give Br radical. This reaction exhibits the opposite regiochemistry as the ionic mechanism.
Edit: Duh, I spaced. NBS gives different regiochemistry. Br2s add directly to the double bond, NBS substitutes at the allylic position, and the alkene remains intact.
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